The formation of Tau amyloid fibers and soluble oligomers are associated with Alzheimer's disease and a host of related tauopathies. The work summarized in this proposal seeks to characterize the ensemble of structural conformations, ranging from toxic to protective, that are accessible to Tau. The atomic resolution structure of the remaining Tau fiber core will be determined from nanocrystals by micro-electron diffraction (micro-ED), allowing a panel of inhibitors that block fibrillization and toxicity to be designed an tested. Proteopathic oligomers that are responsible for seeding the spread of toxic Tau throughout the brain will be mapped biochemically and at atomic resolution to uncover segments that are important for oligomer stability and seeding. Additionally sequences that I identified in the woodpecker Tau homolog, which I hypothesize protect against the formation of toxic Tau aggregates, will be tested in human Tau. Emerging themes identified through the proposed research plan will guide rational inhibitor design efforts underway in the Eisenberg lab to develop D-peptides that block full length Tau aggregation.